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Tuesday, April 26, 2011

So today, I thought I would finally indulge those of you who are in constant puzzlement over how to tell the difference between two of the more common "sand stars" found throughout the Indo-Pacific!

This is a surprisingly common question and its not just my colleagues who repeatedly ask me this -I frequently see these two species mistaken for one another in tropical aquarium and "shell collector" magazines, websites and so on..

Hopefully, the names of these two species can be consistently applied. And that's generally good for everyone in the long run...

Know Your Sand Stars! Convergence between Astropecten and Archaster?
So, first, we are talking about TWO different species from two different families in two entirely separate orders.

I should add the caveat that there's just over 100 species of Astropecten throughout the world and a fair number of them are present in the tropical Indo-Pacific.. So, there may be SOME variation if you are looking at different species of Astropecten wherever you may be reading this..

Archaster in the Archasteridae, Order Valvatida. Found in shallow water throughout the tropical Indo-Pacific . Archaster also makes a point of burying itself into sediment... (yeah-there's no single video that is as good as the one above..so these will have to do!)

In spite of belonging to two completely differentand distant groups of starfish, BOTH live in sandy bottoms with loose, unconsolidated sediment and possess the ability to dig and hide under that sand or sediment.

As a consequence these taxa display a number of superficial similarities that are adaptations for each respective species to live buried in a sandy, muddy or otherwise unconsolidated bottom environment.

In vertebrates, you would see this same kind of relationship-called convergent evolution between dolphins (which are mammals) and fish (which are not). Several consistent morphological features, which appear similar, but that arise in very separate evolutionary groups as adaptations to a particular way of life.

In the case between dolphins and fish-the fins and smooth lemon drop shape are adaptations to life swimming through the water.

In the case of these starfish-the location of the spines and the certain similarities in body shape are independent adaptations to digging through sediment.

These two species are frequently mistaken for one another. and Archaster is apparently quite common and abundant.. So usually, people mistake Astropecten for ArchasterSo, How Do We Tell Them Apart?

1. The Radial Plate series (aka the carinal plates)
So-if there's one immediate and obvious character to use-its the distinctive central row of plates on each arm.Indicated below in blueThis is versus Astropecten which basically has NO distinctive radial line or series of plates... Its just a big jumble...2. Suckered vs. Pointed Tube Feet
Another immediately obvious difference between the two..

Archaster has actual suckered tube feet.... As seen here on this video..

This is as opposed to Astropecten which have pointed or knobby ends on their tube feet..3. Body Shape and Spines on the Outline of the Body.

Archaster only has big flat spines on the lower side of its body that flank the oral surface.. If there are spines on the upper surface, they are small and tiny..Plus..the body itself is shaped rather distinctively. with the rather long-tapering armsThere is more of a prominent angle formed between the surface and the side of the body. Also, the top surface is more flattened...In contast, Astropecten can have broader more triangular arms. This can vary by species-but if you find an Astropecten in the tropical shallow waters of the Indo-Pacific, it will look very similar to this...Arm is more triangular. And many species of Astropecten will have a very prominent perimeter of spines around the body's edge.

There is more of a curved angle formed by the top and the lateral side. And you'll often find the top surface to be more swollen and curved. Not as flat as you would find in Archaster...4. The Spines are Different.
Note that these spines are round and sharp rather than flat and blunt. You can especially see the difference in the abundance and the type of spines on the bottom (oral) surface...In Archaster, the spines are flattened and blunt!!To the seasoned expert, there are actually MORE differences than this. But these were the characteristics that I thought were the easiest to tell apart for those who are simply observing the exterior.

It turns out Archaster is probably one of the more heavily fished starfish species. Not only for aquariums, but one also sees this species on sale for arts & crafts and shell-collector websites and ebay.. but Astropecten turns up as well.. This pic gives you an idea of where this species ends up...

What might the impact be of overfishing these species? The first step is knowing which one is which...

Friday, April 22, 2011

UPDATE! (sorry-late from June 2011) More Giant Pink Star, Pisaster brevispinus pics from BRITISH COLUMBIA! thanks again to Neil McDaniel! Photos by Geoff Grognet

Here with Doug S. for scale....

(From April 2011)
About a week ago Mr. Jim V. of Elma, Washington sent me this pic of a MASSIVE specimen of Pisaster brevispinus-the Giant Pink Sea Star which he collected in a crab net with his sons Josiah and Dylan. (dated April 11, 2011)

So definitely sizeable but until we get exact measurements-its not clear to me if this was larger than the one from BC below, still pretty cool. Based on its feeding biology, it must have some VERY crazy long tube feet!!

Tuesday, April 19, 2011

So, what we're seeing in each of these videos is the ring of feeding tentacles around the mouth of a sea cucumber. That's the "wormy" group of echinoderms that likes to sit around eating sediment and detritus all day!

You can note that in almost all of these the feeding tentacles are moving food from the substrate and "licking" the goodness off into their mouth!!

Tuesday, April 12, 2011

Now, all echinoderms have some kind of unsuual stuff going on-but honestly, how many of them can you look at the shape and ask "Whoa. HOW is THAT thing even ALIVE???"

(get your mind out of the gutter-this doesn't look like that AT ALL!)

This and all of the urchins in this post are sea urchins in the family Pourtalesiidae. These odd sea urchins are found all over the world but only in the deepest of the abyssal seas, usually over 1000 meters (although some occur in less than a 1000 m depth) but several, such as the above Echinocrepis, can occur between 3000 to 5000 m!

There are a number of rather dramatic changes in the body of these urchins-almost seemingly unrecognizable when compared to their more commonly encountered shallow-water cousins...

In your typical urchin-you have the anus on top, mouth on bottom. Most of these have big spines and feed on algae and etc. in relatively shallow water.

And you get some changes from the typical round urchins-and you start asking questions like what end is the front?

the animal now has secondary bilateral symmetry! (with a front and a rear!) This is presumably an adaptation to burrowing..

Mouth is modified to be in front with anus in the rear. (again mouth on bottom-anus on top in your more typical urchin!)

Spines are modified into being finer and more delicate..

but the two most important changes- the test (i.e., the skeleton of sea urchins) is VERY highly modified. How?

a. Its smaller and has MANY strange shapes (see below). b. The physical thickness of the test has become SO thin and SO delicate that in the air its actually FRAGILE and can crumble easily to the touch!Here's a nice spread of the genera (courtesy of the British Museum's Echinoid Database).. As you can see...some of them look like bottles or swollen pieces of fruit.

Almost all of these live by plowing through loosely consolidated deep-sea mud. Only recently have these animals been observed alive with submersibles..One example is that of Cystocrepis in Peru by David et al, 2001 who observed that them plowing through mud AND playing host to a bunch of other commensal invertebrates hanging-on!

But really, there's not a lot known about these urchins...

Its right around here though, that what we know about pourtalesiids starts to get more complicated..

So, uh, Isn't it *difficult* to work on Pourtalesiid Urchins?

You have animals that easily live in THOUSANDS of meters depth on the abyssal plain...
....are seldom encountered

The bodies of these animals are incredibly thin and fragile! Almost as if it were made of crispy paper!

For example, here is what your typical pourtalesiid urchin looks like when you pull it up in a net...you almost have to be some kind of expert with puzzles!!I've heard stories of many a sea urchin expert who has spent a lot of painstaking effort gluing some of these back together!Animal Size Complicates! So you've got a weird-strangely adapted animal..but guess what makes working and recognizing them more of a challenge??

Like most animals-as they grow-they change!

So-a baby looks different from a 15 year old versus a 50 year old vs. someone who is 90. You can see some fairly dramatic changes over this broad age range.

Now, what if you apply that amount of disparity to an animal that looks like a living coke bottle?

On the surface-they look nothing alike-but then you see the FULL transformation-size sequence from Figure 2 of Gage's paper..(note that top series shows specimens viewed from the ventral surface whereas lower series shows lateral aka side view)

And you start to see the complete picture-and how much of it you are missing if you've only got a few specimens..

His study observed 3 different "morphotypes" that is body/color types...from left to right, showing purple (a), tan (b) and white (c) of the pourtalesiid Echinocrepis rostrata.

(Fig. 1 from Vardaro 2010)

Weird looking beasts, eh?

Here's one close up to give you a better idea of what they look like..This is pretty much what these look like when alive (although not with the hole in the top!) and again... VERY fragile..Vardaro extracted and amplified DNA from the 3 morphotypes, compared them, and found that they were all likely representatives of the same species...The white morphotype was consistently smaller than the others, which suggested that it was a likely juvenile..but it was not clear what was responsible for the color difference between the purple and tan variants. These are the sorts of puzzles and mysteries that deep-sea animals make scientists a little crazy. There is a little bit of this that is like working on extraterrestrial life..

Who better to work out something that looks like nothing you'd recognize, than a scientist who understands how nothing you'd recognize would look like?

The west coast of North America is blessed with one of the most diverse faunas of sea stars anywhere in the world and thanks to the convenience of the intertidal and the relatively convenient subtidal depths accessible with SCUBA, a lot of the natural history and marine ecology along the coast has become pretty well known.

So..most species in the genus Solaster live in cold-water areas, such as the deep-sea or near one of the polar regions-Arctic or Antarctic. Based on what's known, most are predators of other animals. Here's one feeding on the deep-sea sea cucumber Pannychia.

Most species of Solaster (and indeed most solasterids) have multiple arms. Their sun-like appearance is where we get their name. "Sol" meaning sun and "aster" referring to the starfish.
One species, Solaster dawsoni lives off the cold-temperate west coast of North America and it has a VERY particular feeding preference.

Starfish Eats Starfish: The 13-Arm Hammerlock!!
So, just how does one starfish (albeit with 13 arms) actually EAT another? What is the attack like?

S. dawsoni roams along the bottom, alternatively raising and lowering the arms which are in front of it.

When contact is made (usually with the tube feet), the animal then begins to bear down the remainder of its arms and etc. and begins a rather huge "lurch" forward!

Solaster dawsoni has now effectively "humped" the prey item and has all of its arms covering over the body.

(Fig. 3 from Mauzey et al. (1968)

To quote the small print in the figure above

"The Solaster has captured the Mediaster (the prey item in the figure)and is winding its rays around those of Mediaster to secure the prey in a "13-arm hammerlock."

The stomach is then applied and begins to digest the prey. Presumably, while its still alive.

Size is often a consideration...and S. dawsoni seems to find an easier time of swallowing smaller individuals, such as smaller sized Pisaster ochraceus and this tiny Leptasterias, which it just swallow whole...

What you can learn by watching "the escape response"..
Prey runs away from the predator.

Among the many animals in the intertidal-observation of the speed and severity of the prey upon contact or even upon "smelling" the predator in the water is considered an important behavioral observation, especially for understanding the interaction between hunters and their prey.

How the prey defend themselves-and how successful they get away also inevitably factors into understanding of how ecosystems work..

Mauzey et al. (1968) below photographed Solaster stimpsoni in its defensive posture relative to an onslaught by S. dawsoni.Solaster stimpsoni essentially tries to push S. dawsoni off it by upending its arms and moving away.

The study by VanVeldhuizen and Oakes (1981) tested the interaction among seven species, including Patiria miniata, Henricia, Leptasterias, Pisaster ochraceus, P. brevispinus, and others. Strong escape responses were observed in ALL of them EXCEPT for one...

and that one species is the "Leather Star" Dermasterias imbricata! Out of 22 trials, exposing S. dawsoni to Dermasterias resulted in the latter doing either nothing or else a "mild" escape response.That "garlic" odor that accompanies Dermasterias may be the deterrant...but the idea has never been fully tested..

Some species, such as the Pink Star- Pisaster brevispinus use tiny, little wrench-shaped structures called pedicellariae to defend themselves. Pedicellariae can cover the surface of the animal and pinch at the tube feet of predators that attempt to "roll" over them..Finally..one of the most interesting reactions was between Solaster dawsoni and the other "big boss" predator of the intertidal/subtidal... Pycnopodia helianthoides aka the Sunflower star.

Many probably know that Pycnopodia is a voracious predator of almost everything that it comes across-sea urchins, mollusks, etc. Pycnopodia's presence in the water can make some mollusks-such as abalone and clams go into some crazy convulsions!Based on lab experiments, it turns out that not only will Pycnopodia RUN upon contact with Solaster-but Solaster dawsoni actually ellicits the *strongest escape response*!!

Out of 35 trials, nearly ALL of the Pycnopodia reacted strongly.

They tested all sizes from 2.5 cm to 23 cm radius and Pycnopodia "immediately fled from S. dawsoni, and at high speed."

And in case you ever wanted to know-the "Retreat speed" for Pycnopodia is about 50 cm/min! (normally about 5 to 10 cm/minute)

A quote from the authors:

Several times we observed that Pycnopodia fled with such haste that many of the tube feet lost contact with the substrate near the end of the power stroke and shot rearwards and upwards....The podia occasionally were pushed back with enough force to "kick" fine particles rearward.

in other words "eat my dust"... The rays on Pycnopodia also autotomize (i.e., they are shed) when Solaster gets a grip on it..

So, here is a video that shows kind of what I mean. Note that the Solaster is applied to the big social cluster of Pycnopodia...then as you watch over the course of the video..they disperse!

All this is a simple reaction to Solaster's mere presence (detected in the water)

But there you have it-a starfish half the size of the formidible Pycnopodia thatwill make the latter gallop off like a scared little kid!

About Me

I pursue starfish related adventure around the world with a critical eye and an appreciation for weirdness.
Support has been courtesy of the National Science Foundation but the views and opinions presented herein are mine and do not reflect the opinions of them or any affiliated institutions.
Need to hire an invertebrate zoologist/marine biologist? Please contact me!